1. Field of the Invention
The present invention relates to a process and apparatus for protecting an alkali metal hydroxide evaporation system against corrosion. In particular, this invention relates to a process of, and apparatus for, impressing a sufficient total protection potential directly upon nickel metal evaporation apparatus during the alkali metal hydroxide evaporation process to cause the apparatus to maintain its state as nickel metal and not form an oxide which can dissolve in the circulating alkali metal hydroxide solution.
2. Brief Description of Art
Caustic soda (NaOH) is commercially manufactured by the electrolysis of brine in chlor-alkali mercury, diaphragm or membrane cells. The aqueous caustic soda product made from the membrane cells contains about 32-34% by weight NaOH, while the aqueous caustic soda product made from diaphragm cells contains about 12-15% by weight NaOH and the product from mercury cells is about 50% by weight NaOH. In order to increase the concentration of NaOH for shipment, the caustic soda products from diaphragm and membrane cells are further concentrated by passing them through evaporation apparatus (commonly referred to as NaOH evaporators). It is desirable to increase the NaOH concentration in these products to about 50% by weight NaOH. Caustic soda made from mercury cell normally does not require evaporation because it is already sufficiently concentrated.
Generally, a portion of these NaOH evaporators are made of nickel metal or nickel alloys. Because the evaporation process is conducted at elevated temperature conditions and with a considerable amount of fluid turbulence in the evaporator system, there is an observed corrosion rate of the nickel or nickel alloy evaporator surfaces and associated piping. This corrosion may result in the concentrated sodium hydroxide product after evaporation having significant amounts of dissolved nickel therein. This amount of corroded nickel in the concentrated product is generally at a concentration of about 0.3 to 0.5 ppm with nickel concentrations running much higher during process upsets and startups. While such contaminated NaOH meets the product specification for Ni of 0.5 ppm for use in many applications, it is undesirable for use in the manufacture of bottled sodium hypochlorite bleach without filtration, which requires a 50% NaOH solution containing no more than 0.3 ppm Ni. This more rigorous specification is required because the Ni impurity catalyzes the decomposition of sodium hypochlorite to sodium chloride and oxygen.
The origin of the nickel corrosion in the NaOH evaporator system is believed due to the oxidation of the nickel in the evaporator equipment to Ni(II) hydroxide by the aerated sodium hydroxide solution, followed by dissolution of a portion of the Ni(II) hydroxide film as illustrated according to the following chemical reactions:
Ni+2OHxe2x88x92xe2x86x92Ni(OH)2+2exe2x88x92
Ni(OH)2+OHxe2x88x92xe2x86x92HNiO2xe2x88x92+H2O
In addition, direct oxidative dissolution of nickel metal may also occur as illustrated according to the following chemical reaction:
Ni+3OHxe2x88x92xe2x86x92NHiO2xe2x88x92+H2O+2exe2x88x92
In order to reduce nickel contamination in these concentrated NaOH products, a number of alternative process improvements have been considered.
These attempts included the addition of a reducing agent to the NaOH evaporator feed. See U.S. Pat. No. 4,585,579 (Bommaraju et al.). Also, the use of magnetic means to remove metal impurities has been contemplated. See U.S. Pat. No. 6,200,455 (Hegeman et al.). And, the use of alternative materials of construction other than nickel metal in the evaporator apparatus has been suggested. See U.S. Pat. No. 3,664,885. And, further, the use of electrochemical cells to treat the resulting NaOH solution after evaporation concentration and thereby remove the metal impurities by reduction onto a suitable cathode in the electrochemical cell. See U.S. Pat. No. 3,784,456 (Otto) and U.S. Pat. No. 3,244,605 (Hotchkiss et al.). And still further, after the NaOH evaporation was completed additional filtration steps have been employed to remove impurities including a portion of these described nickel impurities before the NaOH is used at sodium hypochlorite (bleach) plants.
All of these prior attempts were unduly costly and involved extra processing steps or did not work sufficiently well. Accordingly, the present invention offers a solution to this problem without incurring the undesirable costs and/or additional processing steps required by these prior art attempts.
Similarly, potassium hydroxide solutions may require evaporation to bring their concentrations up to a suitable level for shipment. Thus, the same nickel corrosion problems during evaporation may occur and also require an answer.
The present invention relates to a process for reducing the nickel corrosion of an alkali metal evaporator by the application of total protection potential to the exposed nickel surfaces in order to render the nickel passive to or immune from corrosion during the alkali metal hydroxide evaporation process. In such a process, electrically insulated anodes and cathodes would be inserted into the evaporator system. With a nickel anode and nickel cathode, the following processes would be involved, along with their associated electrode potentials:
E(cell)=xe2x88x921.33 V [Ni(OH)2 reduction at cathode], xe2x88x921.42 V [HNiO2xe2x88x92 reduction at cathode]
This information may be found in Chapter IV, Section 12.3 of the Atlas of Electrochemical Equilibria M. Aqueous Systems by E. Deltombe, DeZouboo and M. Pourboix.
Therefore, one aspect of the present invention is directed to a process for reducing the corrosion of nickel material in alkali metal hydroxide evaporator equipment which comprises the step of impressing a total protection potential directly upon at least a portion of the nickel metal material in the alkali metal hydroxide evaporator that is in contact with an aqueous alkali metal hydroxide solution during evaporation of that solution; said total protection potential being sufficient to reduce the amount of corrosion of the nickel material to an oxide that may dissolve in the alkali metal hydroxide solution.
Another aspect of the present invention is directed to an alkali metal evaporator system that has reduced corrosion of nickel material present in alkali metal evaporator equipment wherein at least one set of an anode and a cathode are attached to the nickel-containing alkali metal hydroxide evaporator system to apply a total protection potential directly upon at least a portion of the nickel material in the alkali metal hydroxide evaporator that is in contact with an aqueous alkali metal hydroxide solution during evaporation of that solution, said total protection potential being sufficient to reduce the amount of corrosion of the nickel material to an oxide that may dissolve in the alkali metal hydroxide solution.
And another aspect of the present invention is directed to a preferred embodiment of this process wherein the total protection potential is maintained within a predetermined range to prevent the formation of hydrogen gas above a concentration of 4% by volume in the evaporator system.
Still another aspect of the present invention is directed to a preferred embodiment of this process wherein a purge gas stream is employed to the alkali metal hydroxide evaporator equipment to (1) prevent the formation of unacceptable hydrogen gas concentrations therein, and (2) allow for a higher total protection potential to be applied to the nickel metal to further reduce any corrosion from occurring.